System and method for using a multi-contact electrode to stimulate the cochlear nerve or other body tissue

ABSTRACT

A system and method for preserving temporal and spatial resolution in complex sounds for poor performing patients having high stimulation thresholds is described. The system and method employs two or more adjacent electrode contacts to deliver concurrent stimulation. This concurrent delivery of stimuli creates a high current field intensity that overlaps between individual current fields generated by the two or more adjacent electrodes and which individual fields are summed to create an overlapping field that has a higher current field intensity than a single current emanating from an individual electrode. The use of this method reduces or eliminates the need to increase either the stimulus current amplitude or to increase the pulse width, both of which may cause loss of system resolution, i.e., loss of fine structure information that is used to resolve complex sounds such as music.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/485,583, filed Jul. 8, 2003, which application is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a system and method for using amulti-contact electrode to stimulate cochlear nerves or other bodytissue. More particularly, the invention relates to systems and methodsthat use multi-channel cochlear nerve stimulation for stimulatingindividuals who have high stimulation thresholds.

Use of implantable cochlear stimulating devices for restoration ofhearing is now a well-accepted modality for treating profound deafness.A cochlear implant system may be fully implantable or partiallyimplantable. In a partially implantable device, there can be twocomponents, an external component containing the battery and animplantable component which contains additional circuitry for processingthe stimulation protocol. The implantable component usually consists ofa stimulating cochlear lead with an array of multiple electrodesattached to the lead. The stimulating lead with the electrode array isinserted into the cochlea, for instance, into the tympanic chamber(scala tympani). After the electrode array is implanted into thecochlea, the electrodes may be stimulated one at time. In multi-channelsystems having independent programmability for each electrode, differentstimulus pulse amplitudes and, in some cases, pulsewidths may bedelivered at two different electrodes in the same time interval.

The stimulation delivered by an electrode is generally a pulse or aseries of pulses. The stimulus pulses are usually biphasic, i.e., thepulses may have a negative first phase and a positive second phase,where the positive second phase is also known as the recharge phase. Thenegative phase and positive phase are charge balanced to preventover-accumulation of charges in the tissue adjacent to the stimulatingelectrode and also to prevent premature corrosion of the stimulatingelectrode. The negative first phase of the pulse has a time duration.This time duration is commonly referred to as the stimulus “pulsewidth”. The pulse width as thus defined does not include the duration ofthe positive second phase.

The stimulation strength or level that just produces stimulation (orcapture) of a nerve is termed a “stimulation threshold”. In cochlearstimulation, the stimulation threshold also correlates closely toperception threshold since the firing of only a few ganglion cells(nerve fibers) can be discerned by an individual. To determinestimulation, a stimulus pulse width (pulse duration) is chosen and heldconstant, for example, 20 microseconds, while amplitude of the pulse isgradually increased. In one method of determining stimulation threshold,the stimulus amplitude is increased until the patient is able toperceive a sound. In an alternative method, the actual neural response(using neural response imaging), or the electrical conduction activityof a cochlear nerve that has been “captured”, may be detected using arecording system when the stimulation threshold has been reached.

The stimulation threshold depends on at least two stimulus parameters:pulse amplitude and pulse duration (or pulse width). The stimulationthreshold curve varies inversely between the pulse amplitude and pulsewidth. Such a threshold curve is also called a strength—duration curve.In accordance with this threshold curve, a larger pulse amplitude maycompensate for a reduction in the pulse width to achieve thresholdstimulation of a nerve. Alternatively, a larger pulse width cancompensate for a smaller pulse amplitude to achieve thresholdstimulation.

Normally, stimulation threshold for cochlear applications may beachieved using a stimulus setting of less than about a 50 microsecondpulse width and a current amplitude less than about 1 milliampere. Insome poor performing patients, however, it may be necessary to increasethe amplitude to the maximum compliance voltage allowed by thestimulator system. These particular patients may be poor performing fora number of reasons. One reason is that disease has caused many nervecells in the cochlea to die. In addition, the patient may have apeculiar anatomical structure that causes the nerves to be locatedfurther away from the stimulating electrodes. As a result, the remainingviable nerves may be dispersed further away from a stimulating electrodeand therefore be more difficult to isolate and stimulate.

Sound information is coded in the auditory system in at least twoimportant ways. The first is temporal coding. Temporal information isconveyed as signal information that depends on the rate of firing(frequency) of a nerve fiber or cell. A stimulus may be repeated as a“pulse train” having a specific firing rate or frequency. The variationof stimulus frequency may be translated to frequency of electricalconduction in a specific nerve that is transmitted to the brain, whichfrequency variation can be perceived as temporal nuances in the sound.Coding of sounds also occurs spatially or spectrally with respect to thearrangement of ganglion cells (nerve fibers) along the cochlea (when thecochlea is viewed as unwound from its coiled state).

The electrode array has a set of electrodes that can be linearly spacedapart along the distal portion of a stimulating lead. As implanted inthe cochlea, the electrode array may be placed adjacent to a particularset of cochlear nerve fibers which line the length of the cochlea(modiolus). The nerve fibers are located between the basal (opening) tothe apical (tip) of the cochlea and are spatially coded such thatcertain sound frequencies preferentially stimulate nerve fibers locatedat the apical ends or the basal end of the cochlea. Thus, by choosing tostimulate through a specific set of electrodes along the cochlea,specific nerve fibers that code for specific sound frequencies can bestimulated. Loudness (intensity) of sound may be coded by recruitingincreasing numbers of cochlear nerve fibers. Thus, a just perceptible orthreshold sound may occur with stimulation of as few as 3 to 10 ganglionnerve cells, whereas to increase the perceived intensity of the sound,hundreds or even thousands of ganglion nerve cells must be recruitedsimultaneously.

The typical solution for stimulating a poor performing patient with highstimulation thresholds is to increase the amplitude of the stimulationuntil the patient reports an adequate loudness percept or untilsufficient loudness is determined. A typical problem is that a patientencounters the stimulation output limits of the device before reachingthe high stimulation levels necessary to achieve adequate loudnesspercept. The applied stimulation may be in the form of either voltagestimulation pulses or current stimulation pulses. The maximum availablesystem stimulation level is reached when the voltage stimulation pulsesor the current stimulation pulses cannot be increased further because amaximum compliance voltage of the stimulation device has been reached.The maximum compliance voltage is termed the system “compliancevoltage.” Increasing the stimulation amplitude, however, can be aninadequate solution because, at such high amplitudes, any “headroom” orextra stimulus amplitude that permits a greater dynamic range ofloudness is eliminated. That is, in order to maintain a greater dynamicrange of loudness, it is desirable to provide stimulation levels belowthe system compliance voltage.

In addition, increasing the stimulus amplitude enlarges the currentfield spread and reduces the spatial specificity or selectivity of anelectrode to stimulate sets of nerve fibers because the larger currentfields between electrodes tend to overlap or “smear” into other spatialregions containing adjacent sets of nerves during different timeintervals. A set of nerves may therefore be stimulated by more than asingle electrode, at slightly different times, resulting in a “smearing”effect.

Another way of increasing the stimulus strength is to increase the pulsewidth. Unfortunately, however, as the pulse width is increased, thetemporal information becomes reduced because the wider pulse durationslimit the maximum rate of stimulation frequency. Generally, a shorterpulse width can operate at a higher stimulus frequency (pulses persecond).

The overall effect therefore in having to increase amplitude or pulsewidth is to lower the resolution of perceived sound.

It would thus be desirable to have a method of stimulation that canmitigate the resulting loss of information in poor performing patients.

What is needed, therefore, is an improved method of stimulating theauditory nerves, or other tissue being stimulated, that retains temporaland/or spectral information for poor performing patients with highstimulation requirements.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing astimulation system and method that retains temporal resolution and, to alesser degree, spatial resolution.

In one aspect of the invention, a method is provided for cochlearstimulation of patients with high stimulation threshold. The methodcomprises: providing a cochlear stimulation system that includes anarray of electrode contacts E1 . . . EN, wherein the number of electrodecontacts N is at least 2, and wherein each electrode contact isindependently programmable; measuring a stimulation threshold includingpulse width and pulse amplitude for at least one of the electrodecontacts; determining whether the measured stimulation thresholdrepresents a high stimulation threshold; and stimulating at least twoadjacent electrodes concurrently with stimulus currents I₁ and I₂, whenthe measured stimulation threshold represents a high stimulationthreshold. I₁ and I₂ are current amplitudes emanating from electrode E1and E2, respectively, which current amplitudes may be the same ordifferent. If different, current steering may be advantageouslyimplemented. Because the two currents may be different, it is preferablethat the cochlear stimulation system have the capability to provideindependently programmable stimulation channels so that the stimulidelivered concurrently through each electrode in the array may beprogrammed individually for pulse width and pulse amplitude.

In one embodiment of the method, the step of predetermining whether ahigh stimulation threshold exists in a poor performing patient can bebased on, for instance, the application of a pulse width that is about50 microseconds and has an amplitude that exceeds 80% of the maximumsystem compliance voltage. The stimulation threshold may be determinedby asking a patient to indicate when a sound is first perceived whilethe stimulus strength is increased gradually stepwise. Alternatively,the stimulation threshold may be determined automatically by thecochlear implant system using a neural response imaging system that cansense the resulting cochlear nerve action potential after asupra-threshold stimulus is applied.

In another embodiment, a method of is provided for preserving temporaland spatial resolution in complex sounds for poor performing patientshaving high stimulation thresholds. The method comprises: providing apatient with an implantable cochlear stimulator (ICS), the ICS havingmeans for programmably providing stimulus currents through selectedmultiple electrode contacts on an electrode array adapted to be insertedinto the cochlea of the patient; determining whether a patient exhibitsa high stimulation threshold; and for those patients exhibiting a highstimulation threshold, simultaneously stimulating two or more adjacentelectrode contacts on the electrode array in order to deliver concurrentstimulation to the cochlea, wherein the concurrent delivery of stimulicreates an overlapping field that has a higher current field intensitythan a single current emanating from an individual electrode contact,and further wherein the concurrent delivery of stimuli reduces oreliminates the need to increase either the stimulus current amplitude orto increase the stimulus pulse width, either of which may cause loss offine structure information that is used to resolve complex sounds.

In another aspect of the invention, a system is provided for preservingtemporal and spatial resolution in complex sounds for poor performingpatients having high stimulation thresholds. The system comprises: animplantable cochlear stimulator (ICS), the ICS having means forprogrammably providing stimulus currents through selected multipleelectrode contacts on an electrode array adapted to be inserted into thecochlea of the patient; means for determining whether a patient exhibitsa high stimulation threshold; and means for simultaneously stimulatingtwo or more adjacent electrode contacts on the electrode array in orderto deliver concurrent stimulation to the cochlea for those patientsexhibiting a high stimulation threshold; wherein the concurrent deliveryof stimuli creates an overlapping field that has a higher current fieldintensity than a single current emanating from an individual electrodecontact, and further wherein the concurrent delivery of stimuli reducesor eliminates the need to increase either the stimulus current amplitudeor to increase the stimulus pulse width, either of which may cause lossof fine structure information that is used to resolve complex sounds.

The delivery of concurrent stimuli from adjacent electrodes in theelectrode array has the effect of summing the current fields to increasethe intensity of the summed activation field in the overlap region. Thesumming of fields produces a current field intensity that is greaterthan the individual current field intensities emanating from any singleelectrode, E1 . . . EN. By thus summing the current fields, the need toboost the stimulus (pulse) current amplitude or the pulse width can beeliminated and hence can circumvent the loss of temporal resolution thataccompanies these compensating tactics. Consequently, substantial soundresolution may be retained in a poor performing patient in addition toproducing adequate sound loudness.

It is thus a feature of the present invention to provide a method ofstimulation in poor performing patients with high stimulation levels,which method utilizes a multi-channel stimulator, having multipleelectrodes and independently programmable channels without sacrificingcompliance voltage headroom.

It is a further feature of the invention to preserve temporal resolutionin poor performing patients.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows, in accordance with the present invention, an illustrationof one embodiment of the cochlear lead with an electrode array formodiolar placement;

FIG. 2 shows, a biphasic stimulus, showing the negative (stimulating)phase of the stimulus and a positive recharge phase of the stimulus;

FIG. 3 shows, a typical strength-duration curve, wherein the curveprovides the threshold stimulus required as paired data points ofstimulus amplitude and pulse width;

FIG. 4 shows a transverse (apical to basal) view of a cochlear nerve andbranching ganglion cells;

FIG. 5 shows a sectional view at line FIG. 5-FIG. 5 of FIG. 4, showing amodiolus surrounded by electrodes;

FIG. 6A shows a sectional view of a modiolus and a surrounding electrodearray with activation fields in a normal performing patient;

FIG. 6B shows a sectional view of a modiolus and a surrounding electrodearray with larger activation fields needed for a poor performingpatient;

FIG. 7A shows, in accordance with the present invention, themulti-electrode method of stimulation for use in a poor performingpatient, which method employs two adjacent electrodes deliveringconcurrent stimuli;

FIG. 7B shows, in accordance with the present invention, another view ofthe multi-electrode method showing activation fields for two differentadjacent electrodes;

FIG. 8A shows, in accordance with the present invention, the method ofstimulation using three adjacent electrodes; and

FIG. 8B shows, another view of the present method, using threedifferent, adjacent electrodes which are stimulated concurrently.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

Before describing the present invention, it will be helpful to reviewthe operation of a typical cochlear stimulation system.

Multi-channel stimulators are used in various implantable medicaldevices. For example, such multi-channel stimulators can be found inspinal cord stimulation devices for treating intractable pain andcochlear devices for restoration of hearing in the profoundly deaf. Asan exemplary application of the present invention, the invention will bediscussed in the context of use in a cochlear implant device. Detailsassociated with the operation of a typical cochlear implant system maybe found in one or more of the following U.S. patents, each of which isincorporated herein by reference: U.S. Pat. Nos. 6,157,861; 6,002,966;5,824,022; 5,603,726; 5,344,387; and 4,532,930.

FIG. 1 shows an illustration of one embodiment of the cochlear lead withan electrode array for modiolar placement.

A representative cochlear stimulation system 10 is illustrated inFIG. 1. A microphone 12 senses acoustic waves and converts such sensedwaves to an electrical signal. The electrical signal is then processedin an appropriate manner by a speech processor (SP) 14. Such processingmay include dividing the signal into different frequency bands andgenerating an appropriate stimulation control signal for each frequencyband. The stimulation control signal(s) is passed on to an implantablecochlear stimulator (ICS) 16 via a radio-frequency communications link15. The ICS 16 is connected to an electrode array 20. The electrodearray 20 is inserted into a cochlea 30. (Note, that the representationof the cochlea 30 shown in FIG. 1 is meant only as a schematicrepresentation.)

The electrode array 20 includes a plurality of spaced-apart electrodecontacts 22 thereon. Each electrode contact 22 is electrically connectedto the electrical circuitry within the ICS 16 by way of respectiveelectrical wires 18 embedded within the electrode array 20 as is knownin the art. The ICS, in response to the control signal(s) received fromthe SP 14, generates an electrical stimulation current on selectedgroupings of the electrode contacts 22.

The cochlea 30, as is known in the art, comprises a snail-shaped memberhaving three parallel ducts that spiral around its center bony region,known as the modiolus. One of the spiraling parallel ducts within thecochlea is the scala tympani. The center bony region, or modiolus, iswhere ganglion nerve cells 32 are located. Each of the ganglion cells 32is coupled to the auditory nerve 40, which connects to the brain.

It is the function of the cochlear stimulation system 10 to electricallystimulate the ganglion cells 22 with electrical stimulation currentrepresentative of the acoustic waves sensed by the microphone 12. Inorder to achieve this function, the electrode array 20 is inserted intothe scala tympani so that the electrode contacts 22 encircle themodiolus and ganglion cells 32. Electrical stimulation current flowsbetween selected electrode contacts 22 and hence stimulates the ganglioncells 32 near the selected electrode contacts, as controlled by the ICS16 in accordance with a programmed or selected speech processingstrategy. The speech processing strategy is defined by the controlsignals received from the SP 14. The control signals are modulated bythe acoustic waves sensed by the microphone 12, thereby causing thestimulation current to stimulate appropriate ganglion cells as afunction of the sensed acoustic waves. For example, low frequencyacoustic waves cause ganglion cells near the apical tip of the cochleato be stimulated, whereas high frequency acoustic waves cause ganglioncells near the basal region of the cochlea to be stimulated.

Stimulation of the ganglion cells can be accomplished using twoelectrode configuration modes. One electrode configuration mode is a“bipolar mode,” which uses two electrode contacts 22 positionedrelatively close to each other. In this mode, the load resistanceappears between the two electrode contacts 22. The load resistance ismade up of the interface between the tissue and electrode contacts 22and the tissue itself between the electrode contacts.

Another electrode configuration is a “monopolar mode,” which employs oneof the electrode contacts 22 in the electrode array and an indifferentelectrode that is relatively distant from the electrode contacts 22. Insome cases, the indifferent electrode can be the exterior container (the“case”) of the ICS, which container can be made from a biocompatible,electrically conductive metal such as titanium. In the monopolarelectrode configuration, the load resistance is made up of the interfacebetween the electrode contact 22 and tissue, the interface between theindifferent electrode and tissue, and the tissue itself between theelectrode contact and the indifferent electrode. Because the monopolarelectrodes are spaced far apart, with more tissue between theelectrodes, the monopolar load resistance tends to be much higher thanthe bipolar load resistance.

In either the monopolar or bipolar electrode configuration mode, thereare two stimulus modes that may be used: a uniphasic stimulus mode and abiphasic stimulus mode. A uniphasic stimulus mode provides current flowin only one direction through an electrode. A biphasic stimulus mode,however, provides current flow in both directions through an electrodewithin a relatively short time period. It is thought that uniphasicstimulation may cause electrical charge to accumulate in the tissue nearthe stimulating electrode and thereby cause injury to this tissue. Inaddition, it is also believed that uniphasic stimulation can causepremature degradation of the electrodes. Therefore, most conventionalmulti-channel stimulators, including ones for cochlear stimulators,typically use some form of biphasic stimulation.

FIG. 2 shows a biphasic stimulus waveform as a function of time. Thebiphasic pulse consists of a negative phase X of the stimulus and apositive, “recharge” phase Y of the stimulus. Stimulus pulse waveforms Xand Y are individually uniphasic. But considered together, they arebiphasic because the flow of stimulation current through an activeelectrode is in both directions. A particular biphasic stimulation inwhich an equal quantity of electrical charge flows in both directionsthrough an electrode is termed a “charged-balanced,” biphasicstimulation. A charge-balanced stimulation can be achieved by ensuringthat the flow of charge in both directions through a stimulatingelectrode is equal over time. As represented in FIG. 2, the accumulationof charge is represented by the area (A) and area (B) below and abovethe zero current flow line, respectively. In the waveform shown in FIG.2, the areas below and above the zero line should be equal over a shortperiod of time in order to achieve charge balancing. Such chargebalancing is believed to prevent injury to cells which are near thestimulating electrode and, furthermore, prevent the stimulatingelectrode from degrading prematurely. For these reasons, conventional,multi-channel stimulators for cochlear implants and spinal cordstimulation generally employ charged-balanced stimulation regimes.

FIG. 3 shows a typical strength-duration curve, wherein the X-Y pairs ofpoints on the curve represent the threshold stimulus curve. Thethreshold stimulus is that stimulus (X,Y pair) that just fires (orcaptures) a target nerve. The X-axis represents the stimulus pulse widthand the Y-axis represents the stimulus amplitude. It can be seen fromthe curve that when a narrow or short duration pulse width is used, thestimulus amplitude must be increased. If a larger pulse width is used,the stimulus amplitude may be made smaller. Thus, the stimulus pulsewidth and amplitude vary inversely.

FIG. 4 shows a transverse (apical to basal) view of the cochlear nerve40 and its ganglion cells (nerve fibers) 32, 32′, 32″, 32′″, 32″″ and32″″′. The spiral line 141 indicates the spiral pathway of the lead andthe approximate placement of the electrode array inside the cochlea. Thecircles 32, 32′, 32″, 32″, 32″″ and 32′″″ shown in FIG. 4 represent theganglion cells, and indicate points of the nerve fibers that may bestimulated by the electrodes. The higher frequencies are spatially(spectrally) mapped to the outer circumference of the modiolus andtoward the basal end 40. The apical portion 110 of the cochlea(modiolus) and regions near the cochlear nerve spatially map the lowerfrequencies in the audible sound spectrum.

FIG. 5 shows a sectional view of the modiolus at the location of lineFIG. 5-FIG. 5 of FIG. 4 (although FIG. 4 does not actually show theedges of the modiolus). An exemplary, three electrode array, havingelectrodes E1, E2 and E3, is shown wrapped around the cochlear nerve.FIG. 5 also shows stimulus current field lines 101, 102 and 103 for acurrent applied to electrode contact E2. Such current field linesindicate current spread at different intensity levels. In most patientsa threshold current I_(threshold), e.g., corresponding to stimuluscurrent field line 101, may be sufficient to stimulate or capture aganglion cell in the cochlea. In poor performing patients, however, whohave fewer viable nerve fibers, it may be necessary to increase thestimulus strength to a value I_(poor), e.g., corresponding to stimuluscurrent field line 103. As represented qualitatively in FIG. 5 by thecurrent field lines 101 and 103, I_(poor) is significantly greater thanI_(threshold) in order to capture enough nerve fibers to be perceived assound by the poor performing patient.

The stimulus strength may be increased by increasing the amplitude ofthe stimulus current, increasing the stimulus pulse width or increasingboth the stimulus amplitude and pulse duration. In accordance with thestimulation threshold strength-duration curve for a ganglion nerve cell,these two parameters may be adjusted to compensate for a deficiency inthe other. Increasing the stimulus amplitude broadens or widens theactivation area in which nerves are present. Similarly, the activationfield may be broadened, to an extent, by increasing the pulse duration(pulse width). Some nerves within the current field will have higherthresholds and may require even a greater stimulus strength to elicitstimulation (capture). Because poor performing patients have fewerviable nerve ganglion cells in any one area, it is often necessary toincrease the stimulus strength to recruit or capture other nearby viableganglion cells and to achieve adequate loudness perception. However, asa result of having to increase stimulus strength, spatial specificitymay also be lost in poor performing patients.

FIG. 6A illustrates a cross-sectional area of the modiolus 200 andelectrode array that is placed in the cochlea in a normal performingpatient. The electrode array, in this particular example, has fiveelectrode contacts E1, E2, E3, E4 and E5 which are spaced equally apartalong the distal tip of a stimulation lead (not shown). Because thestimulation lead is placed inside the scala tympani, the electrodes arealso equally spaced therein. The activation field of a single electrodecontact E2 is represented by field F2. Similarly, electrode contact E3has activation region F3, electrode contact E4 has activation region F4,and electrode contact E5 has activation region F5. A similar activationfield, not shown, exists corresponding to electrode contact E1. It isemphasized that each of these activation fields occupies a space in thecochlea which is offset transversely (perpendicularly) to the planerepresented by the sectional view of FIG. 6A because the lead inside isimplanted in a spiral pathway and hence electrode contacts E1 . . . E5are not in the same plane. It is also important to note that the regionof activation for each electrode only minimally overlaps and encroachesupon the region of activation for an adjacent electrode. In one type ofcochlear stimulation system, only one electrode in the set of electrodecontacts, E1 through E5, is stimulated at any one time. It can be seenthat as each electrode delivers a stimulus, essentially differentpopulations or sets of nerve fibers within each activation field, for agiven stimulus strength, are stimulated at any one time. Whenstimulation thresholds are low, as in a normal performing patient, theregions of activation for each electrode are distinct.

However, in the case of a poor-performing, high-threshold patient, itmay be necessary to increase the amplitude of the current delivered ateach electrode contact. Unfortunately, increasing the stimulus strengthalso makes the activation field associated with one electrode contactencroach into the activation field of another adjacent electrodecontact, resulting in “smearing” of the spectral resolution.

FIG. 6B depicts a modiolus 200 and the same electrode array, E1 . . .E5, as in FIG. 6A, except in a poor performing patient. In such apatient, the current activation field FP1 (activation Field of Poorperformer from E1) is larger than normal. Thus, in order to achievesound perception in the poor performer, it is necessary to stimulate awider region in order to capture enough viable nerve ganglion cells(nerve fibers) within the wider region. As a result, the activationfield for any adjacent pair of electrodes overlap, for example, E1 (FP1)and E2 (FP2). Similarly, the activation fields will overlap between E2(FP2) and E3 (FP3), between E3 (FP3) and E4 (FP4) and between E4 (FP4)and E5 (FP5). Therefore, as each electrode contact from E1 to E5provides a stimulus pulse, in sequence, in different time intervals,certain populations of nerve cells or fibers will be stimulated,sequentially, resulting in a smearing effect and a loss of spatial(spectral) resolution.

Fine structure information in sounds which are coded both spectrally andtemporally in the auditory system are necessary to distinguish complexsounds such as music. In poor performers, the spatial resolution can bediminished because using the larger regions of field activation resultin less spatial specificity. Often, in poor performers, the stimulusamplitude must be increased to the maximum limit of the systemcompliance voltage. In other cases, the available compliance voltage maybe simply insufficient to produce an adequate sound perception. In suchcases, the stimulus strength may be further increased by increasing thepulse width. Such a compensating strategy, unfortunately, can degradetemporal resolution (on top of the already reduced spatial resolution)because use of large stimulus pulse widths prevents the coding of highfrequency information. Thus, in poor performing patients, both spectraland temporal resolution may be degraded.

FIGS. 7A and 7B show, in accordance with the present invention, themethod of stimulation for a poor performing patient. The method employsat least two electrodes to deliver concurrent stimuli and the methodhelps to preserve temporal and, to some extent, spatial specificity.

FIG. 7A illustrates, in accordance with the present invention, themethod of stimulation which uses two adjacent electrode contacts, e.g.,E1 and E2, that concurrently deliver stimuli in one time interval. Inthis example, current activation fields RF1 and RF2 are appliedconcurrently in a poor performing patient. The current fields RF1 andRF2 are summed and the summed activation field has greater intensitythan the individual fields, RF1 and RF2. Consequently, a lowercompliance voltage may be used to provide adequate sound perception inpoor performing patients. It can be seen that the activation fieldcreated by such summation may be more restricted in its effective spacethan either the individual activation fields, RF1 or RF1. In thesimplest form of the present method, the current emanating fromelectrode contacts E1 and E2 may be same. In that case, the overlappingactivation field will be located somewhere between RF1 and RF2. Inanother embodiment of the method of the present invention, the stimulusamplitudes can be different at each electrode contact. For example,there may be two different current amplitudes I₁ and I₂. Suchdifferential, simultaneous delivery of currents at adjacent electrodescan provide current steering such that the overlapping activation fieldcan be deliberately steered closer to one of the activation fields, RF1or RF2, so that a particular region and, hence, a particular set ofganglion cells can be stimulated.

FIG. 7B shows activation fields RF3 and RF4. Thus, in one embodiment ofthe present method, stimulation may occur in the following timesequence: in the first time interval, E1 and E2 can concurrently delivera stimulus; in second time interval, E3 and E4 can concurrently delivera stimulus; and in a third time interval, E5 and E6 can concurrentlydeliver a stimulus. In yet another embodiment, E1, E2, E3 and E4 can alldeliver stimuli simultaneously.

An important advantage of the present method is that temporal resolutionof the system can be preserved because the pulse width does not have tobe increased. Because the pulse width does not have to be increased andsmall pulse widths may be used, e.g., 30 microseconds, the temporalspecificity is preserved. While using two electrodes does decreasespatial specificity, on the other hand, it is an acceptable trade-offbecause the current may be steered. In addition, the spread ofactivation field with two electrodes can result in less smearing than asingle electrode having a large field. Certainly, dynamic headroom canbe preserved, effectively doubling the system compliance voltage.Because both temporal specificity and, to some degree, spectralspecificity are preserved with the present stimulation method, finestructure information can be retained and conveyed.

FIG. 8A shows that the present method can be logically extended tosimultaneous stimulation at three adjacent electrode contacts, e.g., E1,E2 and E3, in which the three overlapping current fields RF1, RF2 andRF3 create a summated activation field F_(sum), which can have a higherfield intensity than any individual component field and can be locatedsomewhere near RF2.

FIG. 8B shows another set of three electrode contacts E4, E5 and E6 thatmay simultaneously deliver stimuli to produce activation fields RF4,RF5, and RF6, respectively. The stimulus amplitude emanating from anyindividual electrode E4, E5 or E6 is much less than an equivalentamplitude emanating from a single electrode.

Thus, in operation, whenever a poor performing patient is identifiedsuch that the current amplitude settings exceeds a predetermined level,e.g., 80% of the compliance voltage of the system, the present methodcalls for delivering concurrent stimulation from at least two adjacentstimulating electrode contacts. By so doing, the temporal and spatialresolution can be preserved using lower stimulus amplitudes and bykeeping pulse width small. In accordance with the present invention, aset of two, three or four adjacent electrodes may be stimulatedconcurrently in a single time interval to achieve current summation. Themultiple electrode contacts, in essence, when stimulated simultaneously,act as a single, large electrode contact, having an activation fieldthat is preferentially nearer to the electrode contacts.

The effectiveness of the present invention has been demonstrated in thelaboratory. Given that a stimulus strength of Xs microamperes deliveredthrough a single electrode contact results in a loudness ranking of R,the equivalent loudness ranking R may be achieved using the method ofthe present invention, by using a stimulus strength that is only Xs/N,where N is the number of adjacent electrodes concurrently delivering thestimulus Xs. N can be a whole number representing up to four adjacentelectrodes. This generalization assumes a stimulation configuration inwhich all electrodes concurrently deliver stimuli having equal energy,i.e., the same amplitude and pulse width.

Thus, as one example, when a 400 microampere stimulus is delivered as abiphasic pulse having a duration of 32 microseconds/phase on electrodecontact E1, a loudness ranking of 6 (on a 1-10 arbitrary scale) isrealized. In accordance with the present invention, when concurrentstimulation is applied through adjacent electrode contacts E1 and E2with a biphasic stimulus pulse width of 32 microseconds/phase, and astimulus amplitude of 200 microamperes, an equivalent loudness rankingof 6 is achieved. As shown by this example, the effective system voltagecompliance is doubled, albeit at the expense of using more electrodecontacts and therefore reducing some spectral specificity. Nevertheless,this is an acceptable trade-off since temporal resolution can beretained and system compliance voltage (dynamic headroom) can beincreased.

Previous techniques of increasing pulse width for poor performingpatients can significantly reduce the temporal information in additionto the loss of spatial specificity due to smearing. In contrast, thepresent method maintains greater temporal information by effectivelyincreasing the available voltage compliance and by obviating the need toincrease the pulse width.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. A method of cochlear stimulation for patients with high stimulationthresholds, comprising: providing a cochlear stimulation system thatincludes an array of electrode contacts E1 . . . EN, wherein the numberof electrode contacts N is at least 2, and wherein each electrodecontact is independently programmable; measuring a stimulation thresholdincluding pulse width and pulse amplitude for at least one of theelectrode contacts; determining whether the measured stimulationthreshold represents a high stimulation threshold; and stimulating atleast two adjacent electrodes concurrently with stimulus currents I₁ andI₂, when the measured stimulation threshold represents a highstimulation threshold; and wherein determining whether the measuredstimulation threshold represents a high stimulation threshold comprisesdetermining whether the pulse width exceeds about 50 microseconds andthe pulse amplitude exceeds about 80% of a system compliance voltage. 2.The method of claim 1, wherein measuring the stimulation thresholdcomprises obtaining feedback from patient perception.
 3. The method ofclaim 1, wherein measuring the stimulation threshold comprisesautomatically measuring the stimulation threshold using a neuralresponse imaging system that includes means for sensing evoked cochlearnerve action potentials in response to application of a supra-maximalstimulus.
 4. The method of claim 1, wherein the stimulus currents I₁ andI₂ are about equal in amplitude.
 5. The method of claim 1, wherein thestimulus currents I₁ and I₂ are different in amplitude.
 6. The method ofclaim 1 wherein the stimulus currents I₁ and I₂ comprise biphasicstimulus currents, having a first phase and a second phase.
 7. Themethod of claim 6 wherein at least a portion of the first phase ofstimulus currents I₁ and I₂ overlap in time.
 8. The method of claim 1,wherein the number of electrode contacts N does not exceed
 4. 9. Amethod of cochlear stimulation for patients with high stimulationthresholds, comprising: providing a cochlear stimulation system thatincludes an array of electrode contacts E1 . . . EN, wherein the numberof electrode contacts N is at least 2, and wherein each electrodecontact is independently programmable to be stimulated with astimulation pulse having a programmed level, and further wherein amaximum stimulation level of the stimulation system is limited by acompliance voltage of the cochlear stimulation system; measuring astimulation threshold including pulse width and pulse amplitude for atleast one of the electrode contacts; determining whether the measuredstimulation threshold represents a high stimulation threshold, whereinthe high stimulation threshold comprises one wherein the measuredstimulation threshold exceeds a predetermined level relative to thecompliance voltage of the stimulation system; and when the measuredstimulation threshold represents the high stimulation threshold,simultaneously stimulating at least two adjacent electrodes; and whereindetermining whether the measured stimulation threshold represents a highstimulation threshold comprises determining whether the pulse widthexceeds about 50 microseconds or the pulse amplitude exceeds about 80%of the system compliance voltage.
 10. The method of claim 9, whereinmeasuring the stimulation threshold comprises obtaining feedback frompatient perception.
 11. The method of claim 9, wherein measuring thestimulation threshold comprises automatically measuring the stimulationthreshold using a neural response imaging system that includes means forsensing evoked cochlear nerve action potentials in response toapplication of a supra-maximal stimulus.
 12. The method of claim 9,wherein the simultaneous stimulation of at least two adjacent electrodescomprises stimulating the at least two adjacent electrodes withrespective stimulus currents that are approximately equal in amplitude.13. The method of claim 9, wherein the simultaneous stimulation of atleast two adjacent electrodes comprises stimulating the at least twoadjacent electrodes with respective stimulus currents that are differentin amplitude.
 14. The method of claim 9 wherein the simultaneousstimulation of at least two adjacent electrodes comprises stimulatingthe at least two adjacent electrodes with respective stimulus currentsthat comprise biphasic stimulus currents, having a first phase and asecond phase.
 15. The method of claim 14 wherein at least a portion ofthe first phase of the respective stimulus currents overlap in time. 16.The method of claim 9, wherein the number of electrode contacts N doesnot exceed 4.